The recovery of hydrogen from hydrogen-hydrocarbon mixtures is an important gas separation in the petroleum refining and related industries. High purity hydrogen is recovered from refinery waste streams containing hydrogen and hydrocarbons with up to four or five carbon atoms, or alternately from synthesis gas generated from natural gas by steam-methane reforming or by partial oxidation of heavier hydrocarbons. Economical recovery of hydrogen from such streams at high purity and recovery often requires a combination of cryogenic distillation or absorption and pressure swing adsorption (PSA), or a combination of diffusion through a polymeric membrane and pressure swing adsorption (PSA). In the latter process combination, a hydrogen-rich permeate is withdrawn from the membrane unit at a lower pressure, is compressed to a higher pressure, and is purified by the PSA system to yield a hydrogen product up to 99.999 vol % purity at a pressure slightly below the higher pressure. Hydrocarbon-rich waste streams from the membrane and PSA units often are used as fuel. U.S. Pat. Nos. 4,398,926, 4,690,695, and 4,701,187 describe various integrations of polymeric membranes and PSA systems for the recovery of hydrogen from various gas mixtures.
Prior art membrane-PSA systems for hydrogen recovery are characterized by a large pressure differential across the membrane as hydrogen selectively diffuses, which requires initial compression to provide a high pressure polymeric membrane feed (typically greater than 200 psig) and recompression of the hydrogen-rich permeate as feed to the PSA system for final purification. These compression steps comprise a significant portion of the capital and operating cost of a polymeric membrane-PSA system for hydrogen recovery.
U.S. Pat. No. 5,104,425 discloses a composite semipermeable membrane comprising microporous adsorptive material supported by a porous substrate, and teaches the use of this membrane for separation of gas mixtures including hydrogen-hydrocarbon mixtures. This membrane differs from conventional polymeric membranes in that the hydrocarbon impurities preferentially diffuse through the membrane and the hydrogen-rich product is withdrawn as a nonpermeate stream at a pressure slightly below the feed pressure.
Improved methods for hydrogen recovery will be needed as the expected demand for hydrogen increases in the petroleum refining, transportation, and related industries. In particular, it is desirable to reduce compression cost and membrane module size when using integrated membrane-PSA systems for hydrogen recovery. The present invention, which utilizes an adsorbent membrane separator integrated with additional gas generation and separation steps as disclosed and defined in the following specification and claims, addresses this need for more efficient methods for the recovery and purification of hydrogen.